That's not true - the frequency spec hasn't changed in any way relevant to what happened. (I think there was some changes to rate-of-change-of-frequency disconn

That's not true - the frequency spec hasn't changed in any way relevant to what happened. (I think there was some changes to rate-of-change-of-frequency disconnect specs for generators and possibly medium-term frequency accuracy, but neither of those things mattered here.) This kind of near-simultaneous failure of two large generators, tripping the low frequency demand disconnect, was always specified to cause a dip in frequency of a level those Siemens trains couldn't cope with. There was actually a very similar failure back in 2008. The only reason that didn't cause such chaos is because those trains hadn't been ordered yet.